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Old 15th November 2010, 02:56 PM   #1
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Default Hum Ho

Newbie here. Be gentle.

I have done a Search and looked through some threads, but it seems that these amps commonly making a humming sound and there are several potential causes. I would like to benefit from the findings and opinions of you guys.

My Yaqin MC100B now makes an unacceptable hum. Both channels. The hum volume increases as the volume pot is turned. Low hum. Not a buzz. It hums on every channel into which a device is plugged, but does not hum when the input selector is turned to a position where no device is connected. When the input selector is turned to a position where a device is connected, the hum occurs and it makes no difference whether or how much input signal is entering. In other words, if a device is turned to "mute," the hum is unchanged.

This amp did not hum a week ago. Two changes I have made that could be affecting this behavior are:

1) This weekend, I relocated my entertainment system about two feet to the left. All devices were removed then reinstalled for this operation. In the meantime, I repaired an electrical outlet behind this setup. The outlet was not being used because, as it turns out, it had an open "neutral" circuit. Fixed that. The outlet tests just fine. 117 volts. The neutral conductor is connected to the ground lug on the outlets. That is.....there is no separate ground conductor, but the chassis of the outlet (and the ground pole) are connected to the neutral circuit. This means ground current would have a path back to the main box. The outlet tester smiles on this arrangement, declaring the outlet to be properly grounded. The outlet that powered this amp last week (with no hum) was wired this way.

2) I adjusted the output tube bias last week. On three tubes, the bias was close to 35 mV. I turned them all to about 55 mV.

The hum, as I say, is unacceptable. Very noticeable, particularly at listening volumes. Any advice you experts can offer would be appreciated. I have a friend whose boat needs a new anchor. That's becoming a reasonable alternative for this unit.
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Old 15th November 2010, 03:18 PM   #2
20to20 is offline 20to20  United States
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If that amp uses a 3-wire plug, the chassis safety ground, and thus the chassis, is now connected to the neutral since you have connected the neutral to the safety ground screw on the outlet. Your shielding may no longer be floating and all the other components may have a connection to the neutral when they may need to be floating too.
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Old 15th November 2010, 03:22 PM   #3
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I can use an extension cord to connect this amp to a true, 3-conductor outlet. I may have done that already. The amp did not hum last week, when it was connected to an outlet in which the ground and neutral circuit are connected. I appreciate the advice, and am doubtful this is the source of the hum.
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Old 15th November 2010, 03:30 PM   #4
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Rosinante View Post
I can use an extension cord to connect this amp to a true, 3-conductor outlet. I may have done that already. The amp did not hum last week, when it was connected to an outlet in which the ground and neutral circuit are connected. I appreciate the advice, and am doubtful this is the source of the hum.
Any outlet that has the safety ground tied to the neutral is creating a very short ground loop instead of a long one that would be formed at the breaker panel. Since the hum is variable with volume control it suggests the hum is coming from the outside and not getting shielded or isolated.
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Old 15th November 2010, 03:34 PM   #5
M Gregg is offline M Gregg  United Kingdom
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Rosinante,

Try disconnecting all input cables to the unit and see if it hums then. If it dosen't try connecting one input at a time. You may have a ground loop.

Regards
M. Gregg
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Old 15th November 2010, 03:35 PM   #6
20to20 is offline 20to20  United States
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Also, house wiring can be paralleled from outlet to outlet and if one is miswired they will all be miswired. The power may be joined at a ceiling light box, there are all sorts of circuit paths that electricians have used to wire a room so be carefull about a quicky safety ground fix you have created.
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Old 15th November 2010, 03:59 PM   #7
20to20 is offline 20to20  United States
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By connecting your safety ground to the neutral you are essentially grounding one side of the tranny primary directly to the chassis.
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Old 15th November 2010, 04:07 PM   #8
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by 20to20 View Post
By connecting your safety ground to the neutral you are essentially grounding one side of the tranny primary directly to the chassis.
20to20,

Did I read correctly has the OP connected the return mains to input Gnd?. Is there no seperate return connection?

I assume there is a neutral return in the mains cable? OP is not using input Gnd as return?

Regards
M. Gregg
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Old 15th November 2010, 04:12 PM   #9
20to20 is offline 20to20  United States
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Quote:
Originally Posted by M Gregg View Post
20to20,

Did I read correctly has the OP connected the return mains to input Gnd?. Is there no seperate return connection?

I assume there is a neutral return in the mains cable? OP is not using input Gnd as return?

Regards
M. Gregg
Quote:
The neutral conductor is connected to the ground lug on the outlets. That is.....there is no separate ground conductor, but the chassis of the outlet (and the ground pole) are connected to the neutral circuit. This means ground current would have a path back to the main box.
Unless I misread this.
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Old 15th November 2010, 04:15 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by 20to20 View Post
Unless I misread this.
Is the Gnd an Earth electrode or combined N & Earth incomming at the breaker board?


Regards
M. Gregg
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